Air traffic control without radar? Those familiar towers, often 40 feet high and topped with 20-by- 10-foot rotating antennas, may gradually give way to ground units the size of dorm room refrigerators.

These devices will be part of a system known as Automatic Dependent Surveillance-Broadcast (ADS-B), and they’re far cheaper than radars to build and maintain. System advocates promise other benefits, such as an increase in the U.S. airspace’s traffic capacity.

Those fridge-size ADS-B ground facilities are virtually devoid of moving parts because, unlike traditional radar units, they have no need to sweep the sky to transmit radio waves and then gauge their return. ADS-B units can be mounted on cell phone towers and many other kinds of structures, and in remote sites that couldn’t logistically accommodate big radars. Test programs are already placing them in the interior of Alaska and on oil platforms in the Gulf of Mexico. Airplanes equipped with ADS-B can also exchange data, conferring to pilots awareness of airspace now available only in the control tower.

ADS-B gets its position information from navigation systems on the aircraft, primarily the satellite-based Global Positioning System, or GPS. The typical airliner’s Flight Management System (FMS) also includes other navigation aids that can back up the GPS, such as the Inertial Reference Unit, which relies on ring-laser or fiber-optic gyros.

To understand the significance of ADS-B, it helps to know a little about what it’s replacing. With the most basic, or primary, radar, a rotating transmitter sends out high-power radio waves, which bounce off the target and return. The system notes where in the 360-degree sweep the target registered, which translates into the target’s azimuth. The time it takes the radio waves to reach the target and return indicates its distance from the transmitter, or its range. With those two coordinates, the target is pinpointed in two-dimensional space.

The primary system is enhanced by Secondary Surveillance Radar (SSR). With each radar sweep, a second, high-frequency signal is transmitted along with the primary. When an aircraft equipped with a transponder receives that signal, the transponder sends out a signal of its own, which registers at the ground station.

The SSR uses that return signal to determine aircraft location much more accurately than the primary system could alone, and it eliminates radar returns from spurious sources, such as birds and terrain. Responses from a Mode A transponder include a four-digit identification code assigned by a ground controller via radio, which pilots update manually during flight. Mode C transponders also transmit altitude information, obtained from the aircraft’s barometric altimeter.

An improved surveillance radar technology is Mode S, for Mode Select. Each Mode S-equipped aircraft has a unique, permanent identification number that remains during the life of the aircraft. It enables the air traffic control computer to tailor its interrogations, addressing only specified targets.

Once an air traffic control computer identifies an aircraft by its address, that aircraft goes into a “roll call.” Subsequent interrogations are transmitted on a schedule. As a result, to track a target, Mode S needs far fewer interrogations than earlier radars, which translates into more accurate position reporting.

A Mode S transponder doesn’t have to wait until it’s prompted from the ground to send out its address. It does so continually, and the unsolicited signals, or “squitters,” can also include readings from the aircraft’s altimeter, plus other flight information. This capability enables new kinds of air-to-air communication, such as the automatic signals of TCAS, the Traffic Alert and Collision Avoidance System, which helps prevent midair

collisions.

ADS-B goes further. At least once per second, the aircraft broadcasts not just ID and altitude but also the other essentials of target tracking, azimuth and range—all without interrogation from the ground.

It does this via an “extended squitter,” using a signal that is longer than most Mode S signals. With the extended squitter, the aircraft can downlink flight details such as airspeed, climb or descent rate, and magnetic heading. Other aircraft and ground stations within about 150 miles receive the information in their cockpits or consoles.

An ADS-B-equipped aircraft includes a Cockpit Display of Traffic Information, showing the pilot a view of neighboring traffic similar to what the controller sees on the ground. Other information is uplinked from the ground, including positions of nearby aircraft not equipped with ADS-B, weather data, and other updates.

Installing ADS-B will usually involve modifying the Flight Management System software and making new hard-wire connections between the FMS and the transponder.

Instead of using extended-squitter transponders broadcasting at 1090 MHz, general aviation aircraft will be equipped with simpler “universal access transceivers,” which broadcast at 978 MHz.

The transceivers will also be installed on airport ground vehicles, because ADS-B functions on the ground as well as in the air.

The development of ADS-B is continuing, with implementation coming in stages over the next decade and beyond as radars gradually are decommissioned. For older aircraft, the cost of conversion to ADS-B would be prohibitive.

So in the meantime, aircraft that do convert will be equipped with a hybrid technology, capable of handling both the old system and the new.

“If they’re interrogated, they will reply to that interrogation, but they’ll also spontaneously broadcast their information,” says Vincent Capezzuto, the FAA’s ADS-B program manager.

One of the issues facing the FAA and its project partners is security. The new system depends on air-to-ground and air-to-air broadcast. In a pure ADS-B environment, an intruder who disables the broadcast capability could essentially become invisible. What’s needed is “some backup surveillance system that would find you even if you don’t want to be found,” says Basil Barimo, vice president of operations and safety at the Air Transport Association. That may require retaining at least some primary radars.

The brass ring in developing ADS-B and related services is a national airspace system that can handle more aircraft.

That probably means an air network that doesn’t depend on radar. Unlike radar, ADS-B’s accuracy does not degrade with distance, so airplanes can fly closer without sacrificing safety.

The FAA plans to make the ADS-B system mandatory for general aviation and transport airplanes by 2014.